Programmable devices are commonly used for changing a circuit after it has been fabricated. One application of programmable devices is in the repair of integrated circuit memories. Integrated circuit memories are often fabricated with several redundant elements (rows or columns) of memory cells. Each redundant element has an associated comparison module which is formed by an array of programmable devices and which is capable of receiving a multi-bit address. The programmable devices of the array are selectively programmed to respond to a specific address as necessary to reroute memory addresses for replacing a redundant element for a failed memory element.
One type of programmable device that is commonly used for these applications is the anti-fuse. In its unprogrammed condition, the anti-fuse functions as a capacitor and presents a very high resistance on the order of 10 Megohms. To program an anti-fuse, its connections are shorted together providing a relatively low resistance path through the anti-fuse, typically presenting a resistance of about 200 to 500 ohms.
Typically, a detection circuit is used to determine the condition of an anti-fuse. The detection circuit includes a pull-up transistor that is connected in series with the anti-fuse between the Vcc rail and the ground rail of the detection circuit A voltage level detecting circuit connects the ungrounded terminal of the anti-fuse at a detection node to the output of the detection circuit. For an unblown condition for the antifuse, a voltage at approximately Vcc is provided at the detection node. For a blown condition for the anti-fuse, the shorted anti-fuse connects ground to the input of the level detecting circuit. A latch arrangement, triggered by the voltage level detecting circuit, is provided to isolate the detection node and the blown anti-fuse from the Vcc rail for the blown condition for the anti-fuse.
In programming an anti-fuse, the anti-fuse may not blow clean. In such case, the anti-fuse presents a relatively high resistance which can be on the order of 400 K ohms. Accordingly, a partially blown anti-fuse forms a voltage divider in combination with the pull-up transistor so that rather than being at ground, the voltage provided at the detection node will trip the level detecting circuit, so that the detection circuit will produce an erroneous output.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a detection circuit which can identify a programmed device, in particular a high resistance antifuse.